MMR vaccination rates among U.S.-born children of Somali descent used to be higher than among other children in Minnesota. But the rates plummeted from 92 percent in 2004 to 42 percent in 2014, state health department data shows, well below the 92-94 percent threshold needed to protect a community against measles.

Wakefield, a British activist who now lives in Texas, visited Minneapolis at least three times in 2010 and 2011 to meet privately with Somali parents of autistic children, according to local anti-vaccine advocates. Wakefield’s prominence stems from a 1998 study he authored, which claimed to show a link between the vaccine and autism. The study was later identified as fraudulent and was retracted by the medical journal that published it, and his medical license was revoked.

The current outbreak was identified in early April. As of Thursday, there were 41 cases, all but two occurring in people who were not vaccinated, and all but one in children 10 and younger. Nearly all have been from the Somali American community in Hennepin County. A fourth of the patients have been hospitalized. Because of the dangerously low vaccination rates and the disease’s extreme infectiousness, more cases are expected in the weeks ahead.